CHICAGO — A former chief of staff to ousted Gov. Rod Blagojevich pleaded guilty Wednesday to taking part in a scheme to sell or trade President Barack Obama's Senate seat and vowed to testify at the ex-governor's corruption trial.

John Harris, 47, admitted in his signed plea agreement that he repeatedly talked with Blagojevich about how the then-governor could turn his power to name a successor to Obama to his own financial advantage — such as trading it for a high-paying job.

Harris's defense attorney, Terry Ekl, said he expects Blagojevich to go to trial on racketeering conspiracy charges and that his client would testify to outline what he knows about corruption in the administration.

Harris pleaded guilty before U.S. District Judge James B. Zagel to one count of wire fraud for a November 2008 phone conversation about appointing Obama adviser Valerie Jarrett to the seat vacated by Obama's election as president.

In his plea agreement, Harris outlined a scheme under which Blagojevich would get a job as head of a union-sponsored organization, Change to Win, in exchange for giving Jarrett the Senate seat. Officials of the Service Employees International Union were to broker the scheme had it gone forward. It did not.

Jarrett, now a White House adviser, was not mentioned by name in the plea agreement but has been identified in the past as the Senate Candidate B who was to get the seat under the plan. Jarrett eventually took her name out of contention for the seat and has been accused of no wrongdoing in the case.

SEIU officials have also not been accused of any wrongdoing.

After Blagojevich believed that Jarrett was no longer a candidate, he discussed two other possible candidates for the Senate seat, the plea agreement said. In one case, he allegedly told Harris that he wanted to receive the candidate's entire campaign fund in exchange for the Senate appointment.